1. Field of the Invention
The present invention relates to the regeneration of catalysts employed in a fluid catalytic cracking process. More particularly, this invention relates to the combustion of volatile hydrocarbons in mixture with spent fluid catalytic cracking catalyst prior to said mixture entering the regeneration zone.
2. Description of the Prior Art
The fluidized catalytic cracking of hydrocarbons is well known in the prior art and may be accomplished in a variety of processes which employ fluidized solid techniques. Normally in such processes, suitably preheated, relatively high molecular weight hydrocarbon liquids and/or vapors are contacted with hot, finely-divided, solid catalyst particles either in a fluidized bed reaction zone or in an elongated riser reaction zone, and maintained at an elevated temperature in a fluidized state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons typical of those present in motor gasolines and distillate fuels.
During the cracking reaction, coke is deposited on the catalyst particles in the reaction zone thereby reducing the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stock. In order to restore a portion, preferably a major portion, of the activity to the coke contaminated or spent catalyst, the catalyst is transferred from the reaction zone into a regeneration zone. Typical regeneration zones comprise large vertical cylindrical vessels wherein the spent catalyst is maintained as a fluidized bed by the upward passage of an oxygen-containing regeneration gas, such as air. The fluidized catalyst forms a dense phase catalyst bed in the lower portion of the vessel and a dilute catalyst phase containing entrained catalyst particles above, with an interface existing between the two phases. The catalyst is contacted with the oxygen-containing regeneration gas under conditions sufficient to burn at least a portion, preferably a major portion, of the coke from the catalyst. Flue gas, which normally comprises gases arising from the combustion of the coke on the spent catalyst, inert gases such as nitrogen from air, any unconverted oxygen and entrained catalyst particles, is then passed from the dilute catalyst phase into solid-gas separation means within the regeneration zone (e.g., cyclone separators) to prevent excessive losses of the entrained catalyst particles. The catalyst particles separated from the flue gas are returned to the dense phase catalyst bed. A substantially catalyst-free flue gas may then be passed from the separation means to equipment downstream thereof, for example to a plenum chamber, or be discharged directly from the top of the regeneration zone. The regenerated catalyst is subsequently withdrawn from the regeneration zone and reintroduced into the reaction zone for reaction with additional hydrocarbon feed.
Commonly, spent catalyst from the reaction zone is passed therefrom to a stripping zone for removal of volatile hydrocarbons from the catalyst particles prior to transferring the catalyst to the regeneration zone. However, the volatile hydrocarbons not recovered as product from the reaction zone will pass with the spent catalyst into the regeneration zone wherein they are combusted in preference to the carbon on the spent catalyst. This results in exhaustion of the oxygen in the regeneration gas in the area where the spent catalyst and volatile hydrocarbons enter the regeneration zone. Normally, the spent catalyst and volatile hydrocarbons enter the regeneration zone at an off-center location to avoid interference with the regeneration overflow well and/or auxiliary heating air section. Thus, one area of the dense phase bed is essentially starved of oxygen such that CO rather than CO.sub.2 will be formed. In contrast, an excess of oxygen is present in the remaining portion of the dense phase bed since volatile hydrocarbons are not present therein.
The CO thus formed in this localized area passes from the dense phase bed into the dilute catalyst phase where it is reacted with oxygen leaving the oxygen-rich portions from other parts of the dense phase bed according to the following equation, an exothermic reaction: EQU 2CO+O.sub.2 .fwdarw.2CO.sub.2 ( 1)
This oxidation of carbon monoxide is commonly referred to as "afterburning" when it occurs in the dilute catalyst phase (see "Oil and Gas Journal", Vol. 53, No. 3, pp. 93-94, 1955, for further discussion). The "afterburning" causes a substantial increase in the temperature of the dilute catalyst phase which may exceed about 1500.degree. F. Such high temperatures in the dilute catalyst phase can cause deactivation of the small amounts of catalyst still present, thereby requiring additional catalyst replacement to the process in order to maintain a desired catalytic activity in the hydrocarbon reaction zone. Additionally, these high temperatures may cause damage to mechanical components of the regeneration zone, particularly in that portion of the regeneration zone in contact with the substantially catalyst-free flue gas wherein the temperature may increase to 1800.degree. F. or greater. Such high temperatures are realized because the reaction shown in equation (1) proceeds rapidly within the substantially catalyst-free flue gas since there is very little entrained catalyst present to absorb the heat released, and thereby reduce the rise in temperature. Thus, in that portion of the regeneration zone wherein the flue gas is substantially catalyst-free, there will occur a rapidly accelerating rise in temperature due to the heat released as complete combustion of carbon monoxide occurs in the absence of any means to moderate the temperature therein.
Thus, in view of the undesirable consequences resulting from the combustion of volatile hydrocarbons in the regeneration zone, it would be desirable to have a simple and convenient method for removing said hydrocarbons prior to their entering said regeneration zone.